Photocatalytic conversion of carbon dioxide and water into substituted or unsubstituted hydrocarbon(s)

ABSTRACT

A method for the production of hydrocarbon(s), such as methane, substituted hydrocarbons, such as methanol, or the production of hydrogen, the method comprising the steps of contacting a first catalyst with water in order to photocatalyse the splitting of at least some of the water into hydrogen and oxygen; and contacting a second catalyst with a gas stream comprising carbon dioxide and at least some of the hydrogen produced from step (a) in order to photocatalyse the reaction between the hydrogen and carbon dioxide to produce hydrocarbon(s), such as methane, and/or substituted hydrocarbons, such as methanol. In an embodiment, the catalyst comprises gold and or ruthenium nanoclusters supported on a substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a § 371 national stage of PCT InternationalApplication No. PCT/AU2016/051175, filed Nov. 30, 2016, which claimspriority to AU2015904952, filed Nov. 30, 2015, the contents of which isincorporated herein in its entirety by this reference

FIELD

The present invention relates to the production of hydrocarbon(s) suchas methane or substituted hydrocarbon(s) such as methanol. In oneembodiment, the hydrocarbon(s) can be formed using water and carbondioxide as precursor materials.

BACKGROUND

For many decades, oil has been the main feed stock for the production ofhydrocarbons. Recently, however, concerns over increases in costs offossil fuels and the effect of global warming have prompted theexploration of alternative more renewable feed stocks.

Carbon dioxide has received much attention as an alternative feed stockfor the production of methane, because there is a drive to reduce carbondioxide emissions to help slow global warming, and because it is cheapand readily available. Carbon dioxide can be converted into hydrocarbonssuch as methane by reacting it with hydrogen, for example via theSabatier reaction. The hydrocarbons produced can then be converted intoother forms such as methanol.

While the process of converting carbon dioxide into hydrocarbons isrelatively well understood, it has been an energy intensive process. Forexample, the hydrogen used for carbon dioxide conversion is usuallyproduced from fossil fuels by steam reforming, and conversion of carbondioxide to hydrocarbons typically requires relatively high temperatures.Catalysts are often employed to increase efficiencies, but they can addsignificant costs to the process. Using fossil fuels to producehydrogen, which is then converted back into hydrocarbons, is known to bea relatively inefficient process. Alternative hydrogen feed stocks, suchas water, can be used, but their use is a relatively energy intensiveprocess.

Accordingly, there is a need to find a more sustainable way of producinghydrocarbons using more efficient and environmentally friendly methods.

SUMMARY OF DISCLOSURE

According to a first aspect of the present invention there is provided amethod for the production of hydrocarbon(s), such as methane, orsubstituted hydrocarbons, such as methanol, the method comprising thesteps of:

-   -   contacting a catalyst with water and carbon dioxide in the        presence of light in order to photocatalyse:    -   (i) the splitting of at least some of the water into hydrogen        and oxygen; and    -   (ii) the reaction between hydrogen and carbon dioxide to produce        hydrocarbon(s), such as methane, and/or substituted        hydrocarbons, such as methanol;    -   wherein the catalyst comprises at least gold and ruthenium, in        the form of at least one nanocluster supported by a support        substrate such as a titanium dioxide substrate.

According to a second aspect of the invention, there is provided amethod for the production of hydrocarbon(s), such as methane, orsubstituted hydrocarbons, such as methanol, the method comprising thesteps of:

-   -   a. contacting a first catalyst with water in order to        photocatalyse the splitting of at least some of the water into        hydrogen and oxygen;    -   b. contacting a second catalyst with a gas stream comprising        carbon dioxide and hydrogen, at least some of the hydrogen can        be produced from step (a), in order to photocatalyse the        reaction between the hydrogen and carbon dioxide to produce        hydrocarbon(s), such as methane, and/or substituted        hydrocarbons, such as methanol.

The first and second catalyst can be the same catalyst. The firstcatalyst and the second catalyst can be different catalysts. The firstand second catalysts can comprise one or more nanoclusters. The firstand second catalysts can be immobilized on the support. The first andsecond catalysts can be activated on the support. The nanoclusters cancomprise gold and/or ruthenium nanoclusters. The nanoclusters can havean average cluster size of less than about 2 nm.

It should be understood that the splitting of at least some of the waterinto hydrogen and oxygen can include splitting the water into hydrogenand or oxygen containing species such as hydrogen radicals, hydroniumand or hydroxyl radicals.

Without wishing to be limited by hypothesis or theory, embodiments ofthe invention will now be summarised and then described based on theunderstanding of how the catalyst performs under various conditions.

A. Embodiments in which there is a First Catalyst and a Second Catalyst

According to a third aspect of the present invention there is provided amethod for the production of hydrocarbon(s), such as methane, orsubstituted hydrocarbons, such as methanol, the method comprising thesteps of:

-   -   a. contacting a first photocatalyst with water in the presence        of light in order to photocatalyse the splitting of at least        some of the water into hydrogen and oxygen;        -   wherein the first photocatalyst comprises gold nanoclusters            supported by a titanium dioxide substrate;    -   b. contacting a second catalyst with a gas stream comprising        carbon dioxide and at least some of the hydrogen produced from        step (a) in order to catalyse the reaction between the hydrogen        and carbon dioxide to produce hydrocarbon(s), such as methane,        and/or substituted hydrocarbons, such as methanol;        -   wherein the second catalyst comprises ruthenium nanoclusters            supported by a titanium dioxide substrate.

In an embodiment, the catalyst can comprise a first catalyst and asecond catalyst. The first catalyst can be a photocatalyst. The secondcatalyst can be a photocatalyst.

Step (a): A First Catalyst for the Photocatalytic Splitting of Waterinto Hydrogen and Oxygen

The first photocatalyst preferred for use in step (a) above can comprisea substrate and an active metal component. The substrate can begraphene, graphite, carbon black, nanotubes, fullerenes, and/orzeolites. The substrate can be a carbon nitrate CxNy. The substrate canbe a metal oxide or nitride. The substrate can be a titania, silicaand/or alumina. The substrate can be barium titanate or perovskite. Thesubstrate can be a titanium oxide. The titanium oxide support substratecan include anatase and/or the commercially available P25. The substratecan be a monolithic. The substrate can have a planar surface such as aplate or disc. The substrate can be particulate. The substrate cancomprise nanoparticles. In one embodiment, the substrate comprisestitanium dioxide nanoparticles.

Photocatalysts are activated by light. The light used can be determinedby the specific type of photocatalysts. For example, some photocatalystscan catalyse a reaction using light with a wavelength over a broadrange, while others may only catalyse the reaction with a specificwavelength e.g. 365=/±5 nm or 400±5 nm. Depending on the reaction, itmay be advantageous that the first photocatalyst comprises two or moretypes of photocatalyst where one can perform at a specific wavelengthand the other can perform over a broad wavelength range Usually, themore intense the light, the more efficient the catalytic process is.However, in some circumstance the reactants and/or products may bedegraded if the light source is too intense. Therefore, it can beadvantageous to have a balance between rate of catalysis and the rate ofdegradation of the reactants/products.

A common wavelength range for photocatalysts are those in theultraviolet range i.e. 200-400 nm. The source of ultraviolet light maybe from a dedicated lamp or may be from a natural light source, such asthe sun. Usually commercial ultraviolet light sources have a greaterIntensity compared to natural sources. Natural light sources can have aUV intensity (i.e. <400 nm) of approximately 4.63 mW cm⁻², whilecommercial sources can be many times more intense, such as >1000 mWcm⁻². Using a natural light source can be advantageous from an energyInput perspective, and can make the process more environmentallyfriendly. If a natural light source is used, it may be supplemented witha commercial light source. Such circumstances may include during timesof inclement weather and/or during times of reduced light activity, suchas at night. In areas with plentiful natural light, e.g. Australia, itmay be advantageous to rely on the sun as a source of ultraviolet lightduring the day and a commercial light source during the night to allowconstant photocatalytic activity over a 24 hour period. Concentratedsolar sources, can provide energies in the range of from about 500 toabout 1000 suns i.e. 2315-4630 mW/cm⁻².

An advantage of using photocatalysts (when compared to other types ofcatalysts) is that they often do not require the use of heat to catalysereactions. Not requiring heat can decrease operational costs, make theproduction of hydrogen more environmentally friendly, and make theproduction of hydrogen safer. Temperatures that can be used forphotocatalysis are around room temperature e.g. about 20-30° C., but maybe as high at about 100-300° C., for example 250° C. Nevertheless, thephotocatalyst could be used with the addition of heat, which may allowfor a reduction in light energy input.

The active metal of the photocatalyst can be selected from gold, silver,copper, platinum, palladium, nickel, rhenium, ruthenium and/or titanium,and/or other transition metals and their corresponding oxides. In aembodiment the active metal is gold. It may be advantageous to have morethan type of active metal, one of which could be gold. Whilst gold isexemplified herein, it should be understood that the invention is not solimited and other active metal nanoclusters could be prepared using thedetails disclosed herein.

The form in which the active metal is associated with the substrate canbe determined by the reaction and/or the reaction conditions of theformation of the photocatalyst. For example, the active metal(s) couldbe present in the form of complexes, nanoparticles and/orclusters/nanoclusters. It may be advantageous to have more than oneactive metal where each metal has a different form. In a preferredembodiment, the active metal is present as a nanocluster.

By way of background, metal complexes have an active metal that issurrounded by one or more ligand(s). The type of ligand(s) can greatlyaffect the performance of the catalyst. One of the ligands can beimmobilised on the surface of the substrate, which can help to preventthe complex from disassociating from the substrate. This can beadvantageous, for example, in helping to recover the photocatalyst oncea reaction is complete. Nanoparticles, on the other hand, can have anaverage size in a range of from about 5 to about 100 nm. The shape andarrangement of the nanoparticles can greatly affect the function as aphotocatalyst. For example, a nanoparticle with a cuboid shape usuallyhas a lower surface area compared to nanoparticles that are rods orribbons, and a lower surface area is usually associated with a decreasein catalyst efficiency. Clusters or nanoclusters (referred to hereininterchangeably unless the context makes clear otherwise), in yet afurther form, refer to a collection or group of two or more active metalatoms, but usually contain less than approximately 200 atoms. Clusterstypically differ from nanoparticles both structurally andelectronically—unique packing of atoms not seen in larger metalparticles and non-plasmonic (Au/Ag)/metallic. It terms of size,nanoclusters are usually considered as being between complexes andnanoparticles. It is to be understood that the number of atoms used todescribe a nanocluster is the average number and there is typically adistribution associated with the average number. For example,nanoclusters containing more than 20 metal atoms can have a distributionof ±10 or more percent e.g. M30±3, MSS5, M100±10. The metals thatcomprise the nanoclusters can comprise ligands. Similar to complexes,any ligands associated with the nanocluster can be used to stabilise thenanocluster and in some circumstances may help to improve theperformance of the nanocluster when as a catalyst. In some cases, it ispreferred to remove any ligands before the compound is used as aphotocatalyst.

The first photocatalyst can have a support that is photoactive. Theclusters can be deposited onto a support capable of adsorbing light ofappropriate wavelength. The cluster plus the photoactive support formsthe photocatalyst. The support can be particulate itself or is can be abulk solid substrate. The bulk solid substrate can be a wafer such as asilicon wafer or a porous silica disk. The first photocatalyst in theform of a paste can be applied to the support. The thickness of theapplied photocatalyst can be varied.

In one embodiment, the photocatalyst comprises a titanium dioxidesubstrate in the form of nanoparticles; the nanoparticles are associatedwith gold nanoclusters. The gold nanoclusters can comprise Au₃ to Au₁₀₁.The gold dusters can be selected from (Ph₃Pau)₃OBF₄, [(AuPPh₃)₃O]PF₆,Au₅(PPh₃)₄Cl, Au₆(PPh₃(BF₄)₂, Au₆(PPh₃MNO₃)₂, Au₆(PPh₃)₆(PF₆)₂,Au₈(PPh₃)₈(NO₃)₂, Au₈(PPh₃)₇(NO₃)₂, Au₉(PPh₃)₈(NO₃)₃,Au₁₀(PPh₃)₅(C₆F₅)₄, Au₁₁Cl₃{(m-CF₃C₆H₄)₃P}₇, Au₁₁(PPh₃)₇(PF₆)₃,[Au₁₃(Pme₂Ph)₁₀Cl₂](PF₆)₃, Au₁₃(PPh₃)₄[S(CH₂)₁₁(CH₃)]₄,[Au₁₃(PPH₂CH₂PPH₂)₆](NO₃)₄, Au₅₅(PH₂PC₆H₄SO₃Na.2H₂O)₁₂Cl₆,Au₅₅(PPh₃)₁₂Cl₆, Au₁₀₁(PPh₃)₂₁Cl₅, where “Ph” is phenyl and “Me” ismethyl.

Once the size of the nanoclusters begins to increase over approximately2 nm, the activity of the photocatalyst may decrease. In an embodiment,the nanoclusters have an average size of less than about 2.5, 2, 1.5 or1 nm. For example, the average size of e.g. Au₁₀₁ can be approximately1.6 nm.

The number of nanoclusters per substrate nanoparticle may depend on thetype of active metal used. In one embodiment, the number of nanoclustersper nanoparticle is at least about 1, 2, 5, 10, 15, 20, 15 or 30. Thepercentage approximate coverage of the nanoparticles with nanoclusterscan be in the range of from about 0.1 to about 10% or more, or at leastabout 0.1, 0.5, 1, 1.7, 2, 3, 4, 5, 6 or 10% or more as a percentage ofthe total available surface area. In one embodiment, the approximatecoverage of the nanoparticles with gold nanoclusters is in the range offrom about 0.17 to about 1.7 wt %.

The first photocatalyst can be pre-treated prior to use. Treatmentmethods can include calcining and/or acid treatment. Acid treatment canbe performed with or without calcining. Where calcining is used, acidtreatment can be performed before or after calcining. It is thought thatacid treatment has an effect on the interaction between the catalystsubstrate and the active metal during preparation of the photocatalyst.

Calcining can be performed at a temperature of at least about 50, 100,200, 300 or 400° C. to remove any residual carbon contamination from thephotocatalyst surface. Calcining can be performed under oxygen and/orhydrogen atmospheres and/or under vacuum. There is thought to be animprovement in H₂ gas production as the first photocatalyst is treatedunder successively harsher conditions. This may be due to the removal ofany ligands from the photocatalyst surface (leaving only the activemetal clusters behind). It is hypothesised that in some embodiments, theremoval of ligands and an increase in cluster size improves thecatalytic performance of anatase-supported Au clusters.

To help ensure all contaminates (including adventitious carbons) areremoved from the first photocatalyst prior to use, it can beadvantageous to expose the first photocatalyst to a vacuum for anextended prior of time. Prior to use the first photocatalyst can be heldunder vacuum for at least about 1, 2, 5, 10, 12 or 15 hours. It ispreferred that the photocatalyst is not exposed to the atmosphere onceit has been held under vacuum.

The step of contacting the first photocatalyst with water can involveexposing all or some of the surface(s) of photocatalyst with water inorder to effect a reaction. The water can be from any source. The watercan be substantially pure, or it can be a part of an aqueous solution.The water used to produce hydrogen can be in liquid form and/or vapourform. In one embodiment, the step of contacting the photocatalyst withwater comprises immersing the photocatalyst in a body of water. Thewater can flow over the first photocatalyst. The flow can be continuous.When liquid water is used, the first photocatalyst may be homogenouslyor heterogeneously distributed in the body of water. Homogenousdistribution may be performed by vigorously mixing the body of water anda first photocatalyst in a fine particulate form. The firstphotocatalyst can be an aggregate that can easily be separated from thebody of water. Heterogeneous distribution may be achieved byimmobilising the first photocatalyst on at least one stationary support.In one embodiment, the first photocatalyst is supported on rods that canbe inserted into the body of water.

In one embodiment, the step of contacting the first photocatalyst withwater Includes allowing a water vapour to come into contact with thefirst photocatalyst. Bringing the water vapour into contact with thefirst photocatalyst can be performed in a variety of ways, for example,continuously flowing water vapour over the first photocatalyst. Thepressure of the water vapour can be varied to achieve the desired result(optimum hydrogen production). Condensation of water vapour can occur ifthe pressure of the vapour is too high. To prevent condensation, theheat of the vapour may be increased, but applying too much heat toprevent condensation may be undesirable. The water vapour may beprovided at below atmospheric pressures. In an embodiment, step (a) isperformed under 20 Torr of water vapour. Additional gases may beincluded with the water vapour. The additional gas may be an inert gas.The inert gas can be argon (Ar). In one embodiment, step (a) s performedunder 280 Torr of argon (Ar).

During the production of hydrogen, oxygen is also produced according tothe following equation (1):2H₂O→2H₂+O₂  (1).

It can be advantageous to remove either hydrogen and/or oxygen to drivethe reaction through favourable thermodynamics. The hydrogen and oxygengases can be collected and stored for use in a subsequent reaction. Thesubsequent reaction can be the reaction of at least some of the hydrogenwith carbon dioxide in an e.g. 4:1 molar ratio of hydrogen to carbondioxide to produce hydrocarbons such as methane. In one embodiment, allof the hydrogen is passed to a further reaction to assist in theproduction of methane.

The amount of hydrogen that can be produced in step (a) can be at leastabout 15, 50, 80, 100, 150, 200, 250, 350, 450, 550, 1000, 1500, 2000 or5000 μmol hr⁻¹ g⁻¹ cm⁻².

Step (b): A Second Catalyst for the Catalytic Reaction of Carbon Dioxideand Hydrogen

The hydrogen produced in step (a) can be used as feed for the productionof unsubstituted hydrocarbons. Hydrocarbons can include C₁ to C₁₀containing compounds such as methane, ethane, propane, butane, pentane,hexane, heptane, octane, nonane, decane, their various isomeric formssuch as n-, iso-, sec- and tert-alkanes, and their respective oxidessuch as methanol and ethanol. More complex hydrocarbons such asaromatics may also be produced. The hydrocarbons produced can be greaterthan C₁₀. The hydrogen can also be used as a feed for the formation of asubstituted hydrocarbon such as methanol, ethanol, propanol, and so on.

Hydrogen can be converted into methane using the Sabatier reaction shownin equation (2):CO₂+4H₂→CH₄+2H₂O  (2)

Hydrogen can be converted into methanol using the following equation(3):CO₂+3H₂→CH₃OH+H₂O  (3).

Industrially, it is understood that these processes require the use ofhigh temperatures i.e. about 200 to 500° C. and typically they requirethe presence of copper-, zinc oxide- and/or alumina-based catalysts.

The step of contacting the second catalyst with carbon dioxide andhydrogen can involve allowing the gas streams to flow over the surface.The amount of gas introduced to the surface of the second photocatalystcan be controlled (in terms of molar ratio) so as to ensure the desiredreaction product. Steps (a) and (b) can be undertaken sequentially astwo separate method steps, or they can be undertaken concurrently.

The second catalyst can be a photocatalyst. The photocatalyst can beactivated by UV wavelengths of light. The second catalyst preferred foruse in step (b) can comprise a substrate and an active metal component.The substrate can be as described above e.g. graphene, graphite, carbonblack, nanotubes, fullerenes, and/or zeolites. The substrate can be anoxide or a nitride. The substrate can be titania, silica and/or aluminaand their oxides. The substrate can be a titanium oxide. The titaniumoxide support can include anatase and/or the commercially available P25.The substrate can be a planar surface or it could be particulate. Thesubstrate can comprise nanoparticles. In one embodiment, the substratecomprises titanium dioxide nanoparticles.

The active metal of the photocatalyst of step (b) can be selected fromgold, copper, silver, platinum, palladium, nickel, rhenium, rutheniumand/or titanium, and/or other transition metals and their correspondingoxides and/or other transition metals and their corresponding oxides. Inan embodiment the active metal is ruthenium. It may be advantageous tohave more than type of active metal. where at least ruthenium ispresent.

The second catalyst can be applied to a support. The support can be aparticulate to increase the surface area, or the support can be solidsubstrate. The solid substrate can be a wafer such as a silicon wafer ora porous silica disk. The second catalyst can be applied to the supportas a layer. The thickness of the layer can be varied.

The form in which the active metal is supported on the substrate can bedetermined by the reaction and/or the reaction conditions. For example,the active metal(s) may be present in forms of complexes, nanoparticlesand/or nanoclusters. These forms of active metal are described inrelation to step (a) above and that description also applies here. Itmay be advantageous to have more than one active metal, with each metalhaving a different form i.e. nano clusters and complexes. In anembodiment, the active metal is present as a ruthenium nanocluster.

In one embodiment, the second catalyst comprises a titanium dioxidesubstrate in the form of nanoparticles associated with rutheniumnanoclusters. The percentage of ruthenium nanoclusters loaded onto thenanoparticles can be at least about 0.1, 0.2, 0.5, 1, 2, 5 or 10 wt %.

The second catalyst can be pre-treated prior use. Treatment methods canInclude calcining and/or acid treatment. To help ensure contaminates areremoved from the catalyst prior to use, it can be advantageous to exposethe catalyst to a vacuum for an extended prior of time. Calcining can beperformed for a period of at least about 1, 2, 5, 10, 12 or 15 hours.The pre-treatment can be at a temperature of at least about 50, 100,200, 300 or 400° C. to remove any residual carbon contamination from thephotocatalyst surface. Calcining can be performed under oxygen and/orhydrogen atmospheres and/or under vacuum. This may be due to the removalof any ligands from the photocatalyst surface (leaving only the activemetal clusters behind). It is hypothesised that in some embodiments, theremoval of ligands and an increase in particle size improves thecatalytic performance of anatase-supported Ru clusters.

A ruthenium-based catalyst may significantly reduce the temperaturesand/or pressures required to produce methane and/or methanol. Forexample, temperatures less than about 100, 200 or 250° C. with pressurebelow a few atmospheres can be used with ruthenium-based catalysts toproduce hydrocarbons (substituted or unsubstituted) from hydrogen. Theefficiency of a ruthenium-based photocatalyst may also be improved byexposure to ultraviolet light. The support may assist in thephotocatalytic production of hydrocarbon or substituted hydrocarbons.

The amount of methane that can be produced in step (b) can be at leastabout 350, 450, 550, 1000, 2000 or 5000 μmol hr⁻¹ g⁻¹ cm⁻².

Apparatus and System

The reaction of steps (a) and (b) may be performed in an apparatus (areactor). The apparatus for step (a) can have an inlet for theintroduction of water. The first photocatalyst of step (a) may be housedin a part of the apparatus and arranged so that the water can come Intocontact with the surface of the first photocatalyst. In someembodiments, the apparatus Is sealable once the water has beenIntroduced. The water can be Introduced as a liquid or vapour. If thewater Is a vapour it can be introduced under pressure. A light sourcecan be arranged Inside or outside of the vessel to allow activation ofthe first photocatalyst. The reaction may be allowed to proceed for aslong as is necessary to produce as much hydrogen as is required (or asis stoichiometrically possible). The temperature and/or pressure withinthe reactor may be slowly increased to effect the optimal reaction. Thegases evolved in the reactor may be collected from the apparatus from anoutlet. The gases may be collected and separated.

A second apparatus may be provided for step (b). In step (b) carbondioxide and hydrogen are mixed at the desired molar ratio in thepresence of a second photocatalyst. The second photocatalyst may behoused in a part of the apparatus and arranged so that the gas streamscan come into contact with the surface of the photocatalyst. In someembodiments, the apparatus is sealable once the gases have beenintroduced. A light source can be arranged inside or outside of thevessel to allow activation of the second photocatalyst. The reaction maybe allowed to proceed for as long as is necessary to produce as muchproduct as is required. The temperature and/or pressure may be slowlyincreased in the apparatus to effect reaction. The gases evolved may becollected from the apparatus from an outlet. The gases may be collectedand separated. In step (a) and step (b) the apparatus can be anautoclave.

In one embodiment step (a) and step (b) are performed in the sameapparatus. Because the production of hydrogen is photocatalytic, it maybe possible to employ both the first photocatalyst and the secondphotocatalysts to produce both hydrogen and hydrocarbons at the sametime, sequentially. The two photocatalysts, first and second, may beindependent of each other, or they may be associated. If the twocatalysts are associated with each other, it may be that, for example,gold clusters and ruthenium nanoclusters are supported on the sametitanium dioxide support. In some embodiments, there are gold rutheniumnanoclusters as described further below. Having one support with twoactive nanoclusters or one support with active Au—Ru nanoclusters mayreduce the operational costs of the production of hydrocarbons and maymake the process more environmentally friendly.

In step (a) the molar ratio of hydrogen to carbon dioxide is alwaysgreater for any carbon dioxide produced during the production ofhydrogen. As the molar ratio of hydrogen to carbon dioxide in Eq. 2 andEq. 3 is always greater than 1:1, any carbon dioxide produced during theproduction of hydrogen is preferably supplemented with an additionalsource of carbon dioxide. If the production of hydrocarbons is coupledwith a production that burns hydrocarbons e.g. for electricity, then theproducts from one process may be a feed stock for another.

B. Embodiments in which there is a Single Catalyst

In this embodiment, it is thought that steps (a) and (b) occur at thesame catalyst site. The method of the present invention can beundertaken in the presence of a catalyst which can

-   -   (i) split at least some of water (into hydrogen and oxygen); and    -   (ii) react hydrogen and carbon dioxide to produce        hydrocarbon(s), such as methane, and/or substituted        hydrocarbons, such as methanol;

The catalyst can comprise a substrate and an active metal component. Thesubstrate can be as described above with respect to the other catalysts.The substrate can be e.g. graphene, graphite, carbon black, nanotubes,fullerenes, and/or zeolites. The substrate can be titania, silica and/oralumina. The substrate can be a titanium oxide. The titanium oxidesupport substrate can include anatase and/or the commercially availableP25. The substrate can be monolithic. The substrate can have a planarsurface such as a plate or disc. The substrate can be particulate. Thesubstrate can comprise nanoparticles. In one embodiment, the substratecomprises titanium dioxide nanoparticles.

The catalyst can be a photocatalyst that is activated by light. A commonwavelength range for photocatalysts are those in the ultraviolet rangei.e. 200-400 nm e.g. 365 nm=/±5 nm. The source of ultraviolet light maybe from a dedicated lamp or may be from a natural light source, such asthe sun. Photocatalysts are described above, and all description madethere applies here unless the context makes clear otherwise. Anadvantage of using photocatalysts (when compared to other types ofcatalysts) is that they often do not require the use of heat to catalysereactions. Not requiring heat can decrease operational costs, make theproduction of hydrogen more environmentally friendly, and make theproduction of hydrogen safer. Temperatures that can be used forphotocatalysis are around room temperature e.g. about 20-30° C., but maybe as high at about 100-300° C., for example 250° C. Nevertheless, thephotocatalyst could be used with the addition of heat, which may allowfor a reduction in light energy input.

The active metal of the catalyst can be selected from one or more ofgold, copper, silver, platinum, palladium, nickel, rhenium, rutheniumand/or titanium, and/or other transition metals and their correspondingoxides. In an embodiment the active metal comprises only ruthenium. Inan embodiment, the active metals comprise gold and ruthenium. The activemetal can comprise gold and ruthenium bound together. The gold andruthenium can have a bond distance in the range of from about 2.5 to 3Å. such as 2.7 to 2.8 Å, or at least about 2.5, 2.7, 2.8 or 3 Angstrom(Å). The gold x to ruthenium y ratio can be about 1:1.5, 1:2, 1:3. Theactive metal can be AuRu₃, Au₂Ru₃ and or Au₂Ru₄. The AuRu₃ can beRu₃AuPPh₃Cl(CO)₁₀. The Au₂Ru₃ can comprise [Au₂Ru₃ (μ-H) (μ₃-COMe)(μ-L₂) (CO₉₎] {where L₂=Ph₂P(CH₂)PPh₂}. The Au₂Ru₄ can comprise [Au₂Ru₄(μ-H) (μ-H) (μ-Ph₂ECH₂E′Ph₂) (CO)₁₂] {where E=E′=As or P; E=As, E′=P}and or [Au₂Ru₄ (μ₃-H) (μ-H) (μ-1,2-Ph₂PC₆H₄PPh₂) (CO)₁₂] and or [Au₂Ru₄(μ₃-H) (μ-H) (μ-dppf) (CO)₁₂] {wheredppf=1,1′-bls(diphenylphosphino)ferrocene}. Sourced from: “MetalClusters in Chemistry: Vol 1 Molecular Metal Clusters”, Editors: P.Braunstein, L. A. Oro, P. R. Raithby. Wiley-VCH 1999. ISBN:3-527-29549-6, the contents of which is incorporated in so far as theAuRu metal clusters are described and unless the context makes clearotherwise.

The active metal can be present in the form of complexes, nanoparticlesand/or clusters/nanoclusters. In a preferred embodiment, the activemetals are present as a nanocluster. Clusters or nanoclusters (referredto herein interchangeably unless the context makes clear otherwise), inyet a further form, refer to a collection or group of two or more activemetal atoms, but usually contain less than approximately 200 atoms. Itterms of size, nanoclusters are usually considered as being betweencomplexes and nanoparticles. The nanocluster can comprise more than 20metal atoms with a distribution of ±10 or more percent e.g. M₃₀±3,M₅₅±5, M₁₀₀±10. The metals that comprise the nanoclusters can compriseligands. Similar to complexes, any ligands associated with thenanocluster can be used to stabilise the nanocluster and in somecircumstances may help to improve the performance of the nanoclusterwhen as a catalyst. In an embodiment, the nanocluster with ligands is ofthe formula Ru₃(μ-AuPPh₃)(μ-Cl)(CO)₁₀. In some cases, it is preferred toremove any ligands before the compound is used as a catalyst. In someembodiments, the ligands assist in the catalytic activity.

Once the size of the nanoclusters begins to increase over approximately2 nm, the activity of the photocatalyst may decrease. In an embodiment,the nanoclusters have an average size of less than about 2.5, 2, 1.5 or1 nm. An active site for reaction can comprise more than one or morenanoclusters.

The catalyst can be applied to a support. The support can be particulateitself or can be a solid substrate. The solid substrate can be a wafersuch as a silicon wafer or a porous silica disk. The first catalyst inthe form of a paste can be applied to the support. The thickness of theapplied catalyst can be varied. The nanoclusters can be supported bye.g. titanium dioxide nanoparticles. The number of nanoclusters persubstrate nanoparticle may depend on the type of active metal used. Inone embodiment, the number of nanoclusters per nanoparticle is at leastabout 1, 2, 5, 10, 15, 20, 15 or 30. The percentage approximate coverageof the nanoparticles with nanoclusters can be at least in the range offrom about 0.1 to 10% or more, or about 0.1, 0.5, 1, 1.7, 2, 3, 4, 5, 6or 10% or more as a percentage of the total available surface area.

In an embodiment, the method can comprise contacting a photocatalystwith water and CO₂ in order to photocatalyse the reaction of water withCO2, wherein the photocatalyst comprises gold nanoclusters and rutheniumnanoclusters or mixed gold-ruthenium nanoclusters supported by atitanium dioxide substrate.

The catalyst can be pre-treated prior to use. Treatment methods caninclude calcining and/or acid treatment. Acid treatment can be performedwith or without calcining. Where calcining is used, acid treatment canbe performed before or after calcining. It is thought that acidtreatment has an effect on the interaction between the catalystsubstrate and the active metal during preparation of the catalyst.

Heterogeneous catalysts and photocatalysts are generally pre-treated insitu before testing, in order to remove advantageous hydrocarbons andother surface-adsorbed species, or to open up catalyst active sites byremoval of ligands. Many different techniques for this can be undertakenfor example including ozone treatment, calcination in O₂ or H₂, andheating under a flow of inert gas. Preferably, any treatment does nothave any damaging effect upon active metal clusters which might causethem or the substrates to which they are attached to aggregate intolarger nanoparticles. The selection of an appropriate pre-treatmentwhich removes adsorbed contaminants while still retaining intactclusters upon the surface for these materials is preferred. Calciningcan be performed under oxygen and/or hydrogen atmospheres and/or undervacuum. Calcining can be performed at a temperature of not more thanabout 50, 100, 200, 300 or 400° C. In an embodiment, the calcining isundertaken at about 200° C. under vacuum. There is thought to beimprovement in H₂ gas production as the catalyst is treated undersuccessively harsher conditions. This may be due to the removal of anyligands from the catalyst surface (leaving only the active metalclusters behind). It is hypothesised that in some embodiments, theremoval of ligands and an increase in particle size improves thecatalytic performance of anatase-supported clusters.

To help ensure all contaminates (including adventitious carbons) areremoved from the catalyst prior to use, it can be advantageous to exposethe catalyst to a vacuum for an extended prior of time. Prior to use thecatalyst can be held under vacuum for at least about 1, 2, 5, 10, 12 or15 hours. It is preferred that the catalyst is not exposed to theatmosphere once it has been held under vacuum.

The step of contacting the catalyst with water can involve exposing allor some of the surface(s) of catalyst with water in order to effect areaction. The water can be from any source and the various ways in whichthe surface of the catalyst can contact water are described above, andalso apply here unless the context makes clear otherwise. The catalystis also exposed to carbon dioxide. Preliminary testing indicates aP_(CO2):P_(H2O) ratio of about 2, 3 or 4 is optimal for solar fuelproduction. In an embodiment, the P_(CO2):P_(H2O) ratio is 3. In someembodiments, optimal production of CO and H₂ was observed at a reagentratio of 1:1, and CO₂:H₂O ratios in the range of at least about 0.5 to4, preferably about 1 to about 3, give peak hydrocarbon production.

During the production of hydrogen, oxygen is also produced according tothe following equation (1):2H₂O→2H₂+O₂  (1).

The hydrogen can be used for the production of unsubstitutedhydrocarbons. Additional hydrogen can be injected into the system ifdesired. Hydrocarbons can include C₁ to C₁₀ containing compounds such asmethane, ethane, propane, butane, pentane, hexane, heptane, octane,nonane, decane, their various isomeric forms such as n-, iso-, sec- andtert-alkanes, and their respective oxides such as methanol and ethanol.More complex hydrocarbons such as aromatics may also be produced. Thehydrocarbons produced can be greater than C₁₀. The hydrogen can also beused for the formation of a substituted hydrocarbon such as methanol,ethanol, propanol, and so on.

Hydrogen can be converted into methane using the Sabatler reaction shownin equation (2):CO₂+4H₂→CH+2H₂O  (2)

Hydrogen can be converted Into methanol using the following equation(3):CO₂+3H₂→CH₃OH+H₂O  (3).

It may be that the catalyst is able to stabilise intermediaries inreaction (1) such as hydrogen radicals, hydronium and orhydroxylradicals that go on to react with CO₂.

The amount of hydrogen that can be produced by the Au—Ru catalyst can beat least about 70, 80, 90 or 100 μmol hr⁻¹ g⁻¹ cm⁻². The amount ofmethane, ethane, ethene, propane and/or propene that can be produced instep (b) can be at least about 350, 450, 550, 1000, 2000 or 5000 nmolhr⁻¹ g⁻¹ cm⁻².

Apparatus and System

The reaction of steps (a) and (b) may be performed in an apparatus (areactor). The apparatus can have an inlet for the introduction of water.The catalyst may be housed in a part of the apparatus and arranged sothat the water can come into contact with the surface of the catalyst.In some embodiments, the apparatus is sealable once the water has beenintroduced. The water can be introduced as a liquid or vapour. If thewater is a vapour it can be introduced under pressure. A light sourcecan be arranged inside or outside of the vessel to allow activation ofthe catalyst. The temperature and/or pressure within the reactor may beslowly increased to effect the optimal reaction. The apparatus can havean inlet for the introduction of carbon dioxide. The catalyst may behoused in a part of the apparatus and arranged so that the carbondioxide can come into contact with the surface of the catalyst. In someembodiments, the apparatus is sealable once the carbon dioxide has beenintroduced. Alternatively the carbon dioxide is continuously introducedinto the apparatus. The reaction temperature can be elevated to at leastabout 120, 150, 180 or 200° C. The gases evolved in the reactor may becollected from the apparatus from an outlet. The gases may be collectedand separated.

According to a second aspect of the invention there is provided anapparatus for the production of hydrocarbon(s) such as methane orsubstituted hydrocarbons such as methanol, the apparatus adapted toundertake the method described herein.

According to a third aspect of the invention there is providedhydrocarbons or substituted hydrocarbons when produced by a method asdescribed herein, or when produced in an apparatus herein described.According to a fourth aspect of the invention there is provided acatalyst when used in the method or apparatus of the invention.

BRIEF DESCRIPTION OF FIGURES

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying non-limiting drawings, inwhich:

FIG. 1: graph showing H₂ gas yield for benchmark Pt—TiO_(a)photocatalysts and control experiments; the latter of which showed no H₂production.

FIG. 2: Bar chart showing a comparison of mean H₂ peak production ratesfor samples that were exposed to vacuum in the reaction cell for 10minutes, compared with those that were evacuated for 12 hours.Acid-washed supports are denoted with the a/w abbreviation.

FIG. 3: Graph showing the number of moles of 02 and CO₂ in the reactioncell throughout the course of an extended experiment, showing theconsumption of O₂ and peak CO₂ production.

FIG. 4: Bar chart showing average H₂ peak production rate for Au₈clusters supported on pure anatase nanoparticles with varioustreatments.

FIG. 5: Bar chart showing average H₂ peak production rate for Au₉clusters supported on pure anatase and acid-washed P25 nanoparticleswith various treatments.

FIG. 6: Bar chart showing average H₂ peak production rate for Au₁₀₁clusters supported on acid-washed P25, treatments. acid-washed anatase,and pure anatase nanoparticles with various treatments

FIG. 7: Bar chart showing a comparison of H₂ peak production rate forAu₈, Au₉, and Au₁₀₁ clusters supported on anatase nanoparticles withvarious treatments.

FIG. 8: Bar chart showing a comparison of H₂ peak production rate forAu₈, Au₉, and Au₁₀₁ clusters supported add-washed P25 nanoparticles withvarious treatments.

FIG. 9: Bar chart showing a comparison of H₂ peak production rate for0.17% w/w Au₁₀₁, Au₉, and Au₈ clusters supported on TiO₂ against 1.0%w/w Pt-P25 and 1.0% w/w Pt-anatase.

FIG. 10: Graph showing hydrocarbon production following photocatalysttreatment at varying calcination temperature. The reaction temperaturewas set at 220° C. for each of the runs.

FIG. 11: early experimental data on ruthenium nanoclusters (Ru₃) in step(b).

FIG. 12: early experimental data on ruthenium nanoclusters (Ru₃) in step(b).

FIG. 13: early experimental data on ruthenium nanoclusters (Ru₃) in step(b).

FIG. 14: early experimental data on ruthenium nanoclusters in step (b).

FIG. 15A-H: early experimental data on gold nanoparticles in step (a).

FIG. 16A: Peak production rates of (left) CH₄ and (right) H₂ by variousphotocatalysts tested. Average production rates are shown from threeindependent tests, with error bars representing the standard error inthe mean of these. Standard reaction conditions were used for all tests:pre-treatment under vacuum at 200° C., reaction at 200° C.,P_(CO2):P_(H2O)=3.

FIG. 16B: Peak production rates of longer-chain hydrocarbon products byvarious photocatalysts tested. Standard reaction conditions were usedfor all tests.

FIG. 17A Peak production rates of (left) H₂ and (right) CH₄ products byvarious photocatalysts tested, normalised to total precious metalcontent of co-catalysts.

FIG. 17B: Peak production rates of longer-chain hydrocarbon products byvarious photocatalysts tested, normalised to total precious metalcontent of co-catalysts.

FIG. 18A: Peak production rates of (left) H₂ and (right) CH₄ by allcluster-deposited titania materials tested here. Standard reactionconditions were used for all tests.

FIG. 18B: Peak production rates of longer-chain hydrocarbon products byall cluster-deposited titania materials tested here.

FIG. 19: Peak production rates of H₂ by all cluster-deposited titaniamaterials, in atmospheres of H₂O/Ar and H₂O/CO₂/Ar. Standard reactionconditions were used for all tests, with CO₂ neglected in the case ofH₂O/Ar atmospheres.

FIG. 20A: Peak production rates of hydrogen and methane by AuRu₃—TiO₂ asa function of combined material pre-treatment and reaction temperature.P_(CO2):P_(H2O)=3 for all tests here.

FIG. 20B: Peak production rates of longer-chain hydrocarbon products byAuRu₃—TiO₂ as a function of combined material pre-treatment and reactiontemperature. P_(CO2):P_(H2O)=3 for all tests here.

FIG. 21A Peak production rates of hydrogen, methane and CO by AuRu₃—TiO₂as a function of reaction temperature. P_(CO2):P_(H2O)=3, pre-treatmenttemperature of 200° C. for all tests here.

FIG. 21B: Peak production rates of longer-chain hydrocarbon products byAuRu₃—TiO₂ as a function of reaction temperature. P_(CO2):P_(H2O)=3,pre-treatment temperature of 200° C. for all tests here.

FIG. 22 shows bond distances in Ru₃(μ-AuPPh₃)(μ-Cl)(CO)₁₀.

EXAMPLES OF EMBODIMENTS OF THE INVENTION

Embodiments of the invention, and other embodiments, will now bedescribed with reference to the accompanying non-limiting examples. Any% referred to herein may be wt % unless the context makes clearotherwise.

Example 1 Formation of the Gold Nonoclusters

An aqueous stock solution of 50 mM gold chloride anions (AuCl₄ ⁻) in aglass vial was made by dissolving HAuCl₄ 3H₂O with the same molar amountof HCl, ensuring stability for more than several months. An aqueousstock solution of 50 mM borohydride anions (BH₄ ⁻) in a glass beaker wasmade by dissolving NaBH₄ granules with the same molar amount of NaOH,guaranteeing stability for several hours at room temperature.

For the smallest nanoparticles of 3.2 nm in diameter, we added 100 μL ofthe AuCl₄ ⁻/H solution to a glass vial with water and later injected 300μL of the BH₄ ⁻/OH⁻ solution all at once, while stirring on a mechanicalshaker for uniform mixing. The total weight of the aqueous solution wascontrolled to be 10 g so that the concentration of gold ions is 0.50 mM.The solution changed colour from light yellow to orange immediately, andthen to red while the vial was stirred for 1 min to release hydrogen gasmolecules. For nanoparticles of other sizes, the amount of the BH₄ ⁻/OH⁻solution was increased from 300 to 650 uL followed by heating for 2-3min at the boiling temperature of water on a hot plate. The averagediameter of gold nanoparticles was precisely controlled from 3.2 to 5.2nm. The amount of the BH4⁻/OH⁻ solution was changed from 200 to 1300 μLduring the search for the “sweet zone” before heating.

Nanoparticles can be prepared by this method as described in the paperentitled: Charged Gold Nanoparticles in Non-Polar Solvents 10 MinuteSynthesis and 2D Self-Assembly, LANGMUIR, 26(10) pp 7410-7417 (2010),the entire contents of which are hereby incorporated by reference intheir entirety. If there are any inconsistencies between this documentand the incorporated document, this document shall take precedenceunless the context makes clear otherwise.

Example 2 Photocatalytic Performance of Pt—TiO₂ for Water-Splitting(Step (a))

In order to establish a benchmark (control) for photocatalyticexperiments of Au/TiO₂, photocatalytic water-splitting experiments wereundertaken using platinised P25 nanoparticles (1.0 wt % Pt/Ti O₂) andplatinised anatase nanoparticles (1.0 wt % Pt/anatase).

In addition, various control experiments were also performed to ensurethat the water vapour was the source of H₂ production. Experiments wereperformed at 28° C. with 20 Torr of H₂O vapour and 280 Torr of Ar in thereaction cell at the start of the experiment, with 20.7 mW cm⁻² of UVlight irradiating the sample disc, equivalent to ˜4.5 suns worth of UVintensity (assuming UV<400 nm).

Selected results of these experiments are presented in FIG. 1. The graphshows that both Pt-P25 and Pt-anatase began to produce H₂ gas after thereaction cell was irradiated with UV light. The blank microfiber discand Pt-P25 without water vapour show no production of H₂ afterirradiation with UV light, indicating that there is only H₂ productionwhen Pt is present on TiO₂ or when Pt—TiO₂ has access to water vapour.Non-platinised P25 and anatase do not show any measurable levels of H₂production (not shown).

For all TiO₂ samples, there is also production of CO₂, but no measurablelevels of O₂ production. This is a consequence of the well-knowncapacity for TiO₂ to photo-degrade carbonaceous species in the presenceof O₂.

Pt-P25 and Pt-anatase have average H₂ production rates of 77.1±9.9 and45.6±12.7 μmol hr⁻¹ g⁻¹ cm⁻², respectively. These results show thewell-known effectiveness of Pt co-catalysts in enabling TiO₂ tophotocatalytically split water. It has been widely accepted that this isdue to decreased electron-hole recombination by allowing for greatercharge separation via migration of the photo-excited electron to Pt. Theunplatinised samples do not produce any notable amounts of H₂ as therate of electron-hole recombination is too high to afford any detectablelevels of H₂, as TiO₂ cannot split water photocatalytically withoutco-catalysts. The increased performance observed for Pt-P25 comparedwith Pt-anatase could be due to the mixed polymorphs of anatase, rutile,and amorphous TiO₂ present in these nanoparticles, which has beendemonstrated to provide a greater degree of charge separation duringphoto-excitation, as well as possible synergistic effects betweenanatase and rutile.

Example 3 Effects of Sample Exposure to Vacuum

Over the course of running control and benchmark experiments, it wasdiscovered that the photocatalytic performance of the catalysts wasimproved when they were prepared under vacuum for an extended period.Examples of the difference in peak H₂ production rates for samplesexposed to vacuum for 10 minutes, compared to those exposed to vacuumfor 12 hours, are shown in FIG. 2. This effect is most pronounced forthe Au/TiO₂ samples, such as Au₉/acid-washed P25, which has a H₂production rate of 166.9±42.3 μmol hr⁻¹ g⁻¹ cm⁻² for those samples thatwere exposed to vacuum for 10 minutes, compared with 511.4±51.1 μmolhr⁻¹ g⁻¹ cm⁻² for those that were exposed for 12 hours.

Given that exposing a sample in the reaction cell to vacuum for 12 hoursprevented the use of the experiment for other samples, attempts weremade to prepare samples under vacuum in a secondary stainless steel cellpreparation cell, evacuated overnight, using the same vacuum line as thereaction cell. This secondary cell did not have the features of theprimary reaction cell and was only used for the preparation of samplesunder vacuum. The samples would then be transferred from the secondarycell and into the main reaction cell as rapidly as possible, takingapproximately 5 minutes for sample changeover. However, this stillresulted in decreased catalytic performance due to the brief exposure tothe atmosphere during sample transfer. There must therefore be someeffect on the catalysts after exposure to an oxidising environment, evenfor a short period, compared with those samples that were evacuatedwithin the reaction cell overnight. After this discovery was madeapparent for a number of samples, all future samples were prepared forphotocatalysis experiments by placing them in the reaction cell andevacuating the cell overnight, then performing experiments withoutexposing the sample to the atmosphere. Only those samples that have beenprepared in this way have been included in the results presented herein.This is similar to most literature studies that undergo rigorous samplepreparation procedures, such as extended flushing of reaction cells withAr or baking samples under UHV for prolonged periods.

Example 4 Effects of Cluster Size

Once the size of the nanoclusters begins to increase over approximately2 nm, the activity of the photocatalyst appears to decrease. It isthought that as the size of the Au nanocluster increases, the energylevels required for hydrogen production begin to match those of thesubstrate. This can be seen in FIGS. 15a-h . Various Au nanoparticlessamples, with a size of approximately 3-5 nm in size respectively, andwhich have had various pre-treatments were tested. For the various Aunanoparticles, pre-treatment before hydrogen production includes nopre-treatment (FIG. 15a, f ), calcining at 200° C. followed by vacuum(FIG. 15b-d ), calcining at 200° C. (FIG. 15e ), and calcining 200° C.in the presence of oxygen (FIG. 15g, h ). The rate of hydrogenproduction was usually less than approximately 160 μmol hr⁻¹ g⁻¹ cm⁻².

The Au₁₀₁ nanoclusters used in the following experiments have a size ofapproximately 1.4 nm and have a much increased hydrogen productionyield.

Example 5 Production of CO₂ and Consumption of O₂

During the water-splitting photocatalysis experiments, the increase inH₂ present in the reaction cell upon UV irradiation was accompanied byan increase in CO₂, and a decrease in O₂, as shown in FIG. 3. It shouldalso be noted that the increased catalytic performance observed betweendifferent samples manifest as both an increase in the H₂ production rateand an increase in the CO₂ production rate.

Studies that deal with the photo-oxidation of organic contaminantspresent on the TiO₂ surface have shown the production of CO₂, and thishas been suggested to be due to photo-activated oxygen. These studieshave shown that even after rigorous steps were taken to dean the TiO₂surface, that even under UHV conditions, there are still carboncontaminants present, which react with oxygen in the system whenirradiated with UV light to produce CO₂ (vide infra).

One of the most likely sources of carbon in the reaction cell is that ofadventitious carbon. This is usually a thin layer of carbonaceousmolecules that are found on the surface of any material or vacuum systemexposed to the atmosphere. It consists primarily of short chainhydrocarbons and small amounts of single and double bonded,functionalised groups

Example 6 Photocatalytic Performance of Au Clusters on TiO₂ forWater-Splitting (Step (a))

Au₈, Au₉, and Au₁₀₁ clusters were supported on P25 and anatasenanoparticles with various treatments as summarised in Table 1.

TABLE 1 A table summarising the different pre- and post-treatmentsapplied to the various supported Au clusters used in photocatalyticexperiments. Cluster Pre- Clusters Per Approximate Type SupportTreatment Post-Treatments Nanoparticle Coverage (%) Au₈ Anatase NoneUntreated 25.56 5.21 O₂ 200° C. O₂ + H₂ 200° C. Au₉ Anatase NoneUntreated 22.71 4.62 O₂ 200° C. P25 Acid washed Untreated 14.31 4.13Heat treated 200° C. O₂ 200° C. Au₁₀₁ Anatase none Untreated 2.02 0.81O₂ 200° C. O₂ + H₂ 200° C. Anatase Acid Untreated 2.02 0.81 washed O₂200° C. O₂ + H₂ 200° C. P25 Acid Untreated 1.28 0.72 washed Heat treated200° C. O₂ + H₂ 200° C.

TABLE 2 A table sowing a summary of the key trends in ligand loss andagglomeration observed for Au8, Au9, Au11 and Au101 on acid-washed P25and pure anatase supports under the various post-treatment conditions.Acid washed P25 Nanoparticle Supported Pure Anatase NanoparticleSupported Treatment Au₈ Au₉ Au₁₁ Au₁₀₁ Au₈ Au₉ Au₁₀₁ Untreated VirtuallyUnchanged Small increase in Partial ligand removal Partial ligandparticle size Half of clusters remain intact, while the removal otherhalf undergo partial agglomeration Unknown if agglomeration occurs, butlikely Washed at Removal of a fraction of clusters from the surfaceUnknown changes N/A 100° C. A portion of clusters remain virtuallyunchanged to size Some removal of ligands Loss of a signifi- Nosignificant agglomeration cant amount of Formation of Au—O bonds, to agreater extent for Au₁₁ clusters from the surface No Au—O for- mationHeated at Agglomeration of a portion of clusters while still ligand-Further agglomer- N/A 200° C. protected (these are still smaller thanAu₁₀₁) ation of clusters, Other portion of clusters lose some ligandsand form but still nanopar- Au—O and agglomerate to larger particlesticulate in nature Of the portion that loses ligands, some clusters maynot Removal of some agglomerate ligands and for- Removal of ligands isless effective for Au₉ mation of Au—O bonds Calcined Increasedagglomeration and size distribution Removal of Removal of Removal of O₂at ligands and ligands and ligands has 200° C. agglomer- agglomer-progressed ation has ation has further progressed progressed Unknown iffurther further agglomer- Half of clus- No fraction ation occurs, tersmaintain of clusters but likely their size maintain their size CalcinedN/A Complete ligand removal Complete li- O₂/H₂ at Agglomeration hasprogressed further, no gand removal 200° C. evidence of any clustersremaining intact. Unknown if agglomeration progresses further, butlikely.

Table 2 summarises the key changes to the physical properties of thesecatalysts due to the various treatments. In general, there is a trend ofligand loss and agglomeration with successively harsher post-treatmentconditions. This effect is far more pronounced for clusters supported onpure anatase nanoparticles than on the acid-washed P25 nanoparticles,showing the strong effect of acidic pre-treatment on the interactionbetween the TiO₂ surface and Au clusters. For samples on either support,there is general evidence for two cluster states after post-treatment,with one portion remaining unchanged, while the other undergoes somelevel of agglomeration.

Example 7 Photocatalytic Performance of the Au₈ Cluster

The peak H₂ production rates for Au₈/anatase with various treatments areshown in FIG. 4. The Au₈/anatase samples have peak H₂ production ratesof 17.92±3.22, 51.74±5.17, and 71.12±7.11 μmol hr⁻¹ g⁻¹ cm⁻² for theuntreated, calcined at 200° C. under O₂, and calcined at 200° C. underO₂+H₂ treatments, respectively. There is a clear improvement in H₂ gasproduction as the clusters are treated under successively harsherconditions.

Calcination at 200° C. under O₂ for Au₈/anatase results in almostcomplete removal of ligands and agglomeration (See Table 2), whileharsher calcination under O₂ followed by H₂ results in complete removalof ligands. Given this information, it is the exposed Au₈ clusters thatare more effective catalysts for photocatalytic water-splitting,compared to untreated Au₈/anatase, of which there is only partialremoval of ligands. However, the loss of ligands may not be the primarycause of increased catalytic activity, given that these calcinationtreatments bring about agglomerated clusters, which do not maintaintheir Au₈ size. It could therefore be argued that the small size of theAu₈ clusters are not beneficial for photocatalytic water-splitting, withlarger Au nanoparticles on the anatase surface yielding the bestcatalytic environment. This is further supported by the deposition ofAu₈ on anatase without any treatment, which also results in some loss ofligands, and a small fraction of Au₈ clusters agglomerating. Given thelow, but still present catalytic activity of these untreated samples,this is further evidence that the agglomerated Au₈ clusters are thecatalytically active sites.

Example 8 Photocatalytic Performance of the Au₉ Cluster

The average H₂ production rates for Au₉ supported on anatase andacid-washed P25 nanoparticles with various treatments are shown in FIG.5. The Au₉/anatase samples have H₂ production rates of 33.5±3.35 and112.9±12.3 μmol hr⁻¹ g⁻¹ cm⁻² for untreated and calcined under O₂samples, respectively. The acid-washed P25 supported samples yield H₂production rates of 82.7±8.27, 511.4±51.1, and 75.3±7.53 μmol hr⁻¹ g⁻¹cm⁻² for the untreated, heat treated under vacuum, and calcined under O₂samples respectively. There is a clear improvement in productivity forthe Au₉/anatase clusters after calcination under an O₂ atmosphere,whereas for the Au₉/acid-washed P25 dusters, there is a large increasein performance after heat-treatment at 200° C. under vacuum, followed bya decrease in performance after further calcination at 200° C. under anOz atmosphere.

Calcination at 200° C. under O₂ for Au₉/anatase results in a largedegree of ligand removal and agglomeration, with only a fraction of thedusters maintaining their size. Given that this support is notacid-washed, this effect is likely more severe than the same treatmenton the acid-washed P25 nanoparticles. This treatment for theanatase-supported Au₉ clusters yields a higher production rate than theuntreated or calcined under O₂ at 200° C. treated, acid-washed P25supported Au₉ clusters, possibly due to the increased size and removalof ligands.

It is therefore interesting that the untreated Au₉/acid-washed P25 has agreater performance than untreated Au₉/anatase, as the former results invirtually no change in the size or ligand coverage of the Au₉ dustersafter they are supported. Heat treatment at 200° C. results inagglomeration of a portion of the Au₉ dusters while stillligand-protected, while the other portion lose some ligands, formingAu—O bonds, and begin to agglomerate. There is also evidence that of theportion that loses ligands, some may not agglomerate. Furthercalcination at 200° C. under O₂ for Au₉/acid-washed P25 results inincreased agglomeration according to HRTEM, but there is no XPS data toprovide details about ligand removal or bond formation between thecluster and the surface. Without wishing to be limited by theory, it isspeculated that the decrease in performance observed for this treatmentcould imply that there is an ideal size for the Au clusters whensupported on acid-washed P25, whereby a small amount of Au₉agglomeration and ligand loss is necessary for ideal performance.Alternatively, it could be the small portion of Au₉ clusters that havelost ligands without agglomerating that are the most effective, as theseare most likely lost with further calcination at 200° C. under O₂. It isdifficult to determine if the large differences in performance betweenthe anatase and P25 series is due to the pure vs mixed polymorph natureof the support, or if it is due to the acid-wash pre-treatment, withoutfurther characterisation studies.

Example 9 Photocatalytic Performance of the Au₁₀₁ Cluster

The average H₂ production rates for Au₁₀₁ supported on anatase and P25nanoparticles with various treatments are summarised in FIG. 6. For theAu₁₀₁/untreated anatase series, the production rates are 50.93±13.3,94.3±15.7, and 190.4±29.6 μmol hr⁻¹ g⁻¹ cm⁻² for the untreated, calcinedunder O₂, and calcined under Oz and H₂ samples respectively. For theAu₁₀₁/acid-washed anatase series, the production rates are 534.8±53.5,112.2±11.2, and 122.7±11.2 μmol hr⁻¹ g⁻¹ cm⁻² for the untreated,calcined under O₂, and calcined under O₂ and H₂ samples respectively.For the Au₁₀₁/add-washed P25 series, the production rates are238.3±16.5, 437.5±43.7, and 188.8±34.0 μmol hr⁻¹ g⁻¹ cm⁻² for theuntreated, heat treated, and calcined under O₂ samples respectively.

The trend of increasing H₂ production for Au₁₀₁/untreated anatase can beattributed to the increased amount of ligand removal and possibleagglomeration under successively harsher calcination conditions. Thiseffect is the same as that discussed previously for both Au₈ and Au₉,supporting the hypothesis that removal of ligands and certain clustersize improves the catalytic performance of anatase-supported Au dusters.

For the Au₁₀₁/acid-washed P25 series, there is a clear maximum inperformance for the heat-treated sample, followed by a decrease withharsher calcination under O₂, similar to the trend observed for Au₉previously. Nevertheless, there is no evidence for a bimodaldistribution of clusters; instead, all clusters have ligands removed andagglomerate, while forming Au—O bonds to the surface. Furthercalcination under O₂ results in a decrease in H₂ production, with anincrease in particle size evidenced by HRTEM. Also of note is that theuntreated samples are more effective than those supported on pureanatase, but less effective than those supported on acid-washed anatase.There is evidence to suggest that support of untreated Au₁₀₁ onacid-washed P25 results in virtually no change to the size or ligandcoverage of the clusters.

For the Au₁₀₁ clusters supported on acid-washed anatase, the highest H₂production rate is observed for the untreated Au₁₀₁ sample, which is thehighest of all samples in this series. The H₂ production rate dropssignificantly after harsher calcination treatments. There is nocharacterisation data available for this series of clusters, thereforeit is unknown if the untreated Au₁₀₁ clusters are maintaining their sizeafter deposition on acid-washed anatase, similar to what occurs foruntreated Au₁₀₁ on add-washed P25, or if there is partial removal ofligands as seen for untreated anatase supports. Given that pre-treatmentof the TiO₂ surface with acid should help to reduce agglomeration, theformer seems more likely. If this is the case, then there is a clearperformance advantage to keeping the Au₁₀₁ cluster intact on theacid-washed anatase surface. Nonetheless, there is the possibility of asmall amount of ligand removal (given the likelihood of this occurringon anatase) comparable to that observed for heat-treated Au₁₀₁ onacid-washed P25, resulting in the similar production rate observed forclusters that maintain size, but have partial ligand removal. This willneed to be confirmed with further characterisation experiments in thenear future.

Example 10 Overall Comparisons of the Photocatalytic Performance BetweenOusters

FIG. 7 shows a comparison between Au₈, Au₉, and Au₁₀₁ clusters supportedon anatase nanoparticles. Similar trends are observed for all threeclusters as successively harsher post-treatments are applied. Whensamples are calcined under an O₂ atmosphere, their H₂ production rateincreased compared to their untreated counterparts. When samples arecalcined under O₂ and H₂, a harsher and prolonged calcination, their H₂production rate is increased beyond that of samples calcined under O₂alone. There is no data available for Au₉ calcined under O₂ and H₂,although it can be assumed that it would follow the same trend as theother clusters, given that Au₉ calcined under O₂ has a production ratewithin experimental error of the production rate for Au₁₀₁ calcinedunder O₂.

As shown in Table 2, these successively harsher treatments result inincreasing ligand removal and agglomeration for all clusters on theanatase surface. It is clear that it is the large, unligated Auparticles that have lost their defined size, which are the mosteffective photocatalysts on the unwashed anatase nanoparticles. Sincethe overall performance also increases when comparing the set of Au₈ toAu₉ to Au₁₀₁ samples, this is further evidence that it is the largest Auparticles that are the most effective photocatalysts on this support.

FIG. 8 shows the similar trends in H₂ production rates for both clustersafter treatment, whereby 200° C. heat treatment of the clusters resultsin a large increase in performance compared to the untreated samples,followed by a decrease in performance for the calcined under O₂ samples.

For the untreated samples, it is known that the clusters remainvirtually unchanged after being supported on acid-washed P25; therefore,the intact Au₁₀₁ clusters are more effective photocatalysts for waterphotolysis than Au₉. This could be due to their larger particle size,similar to the effect observed for Au₁₀₁ supported on pure anatasenanoparticles.

Comparison between these two clusters also reveals that the untreatedAu₁₀₁ clusters do not form any Au—O bonds with the surface; untreatedAu₉ has a portion of clusters forming Au—O bonds with the surfaceaccording to XPS, while untreated Au₉ has a portion of clusters formingAu—O bonds.

The production rate of the heat treated samples are within theexperimental error of each other, and the size measurement by the HRTEMare also within sampling error of each other (2.4±1.7 vs 3.2±1.7 nm forAu₉ and Au₁₀₁, respectively). Therefore, the similar production ratemeasured for these two clusters on acid-washed P25 with the sametreatment could be because the two samples are of similar size afteragglomeration, while still being protected by a comparable number ofligands.

The large drop in production rate for Au₉ calcined under O₂ issurprising given that the size of the nanoparticle is now the same asthat of heat-treated Au₁₀₁ according to HRTEM (3.1±2.1 vs 3.2±1.7 nm forAu₉ and Au₁₀₁ respectively). There is no XPS data for this treatment,but it could be assumed that the extent of Au—O bond formation hasincreased, following the increase observed for the heat-treated samples.Since untreated Au₁₀₁ has no Au—O bonds and performs better thanuntreated Au₉ with Au—O bonds, and that Au₁₀₁ calcined under O₂ may havean increased amount of Au—O bond formation, it is feasible that highlevels of Au—O bond formation is detrimental to the photocatalyticperformance. This is further evidenced by Au₁₀₁ calcined under O₂performing worse than the untreated Au₁₀₁ samples.

Example 11 Photocotalytic Performance of Au₈, Au₉, and Au₁₀₁ Compared toPt—TiO₂

Given that TiO₂ is cheap and relatively abundant, while both Au and Ptare expensive elements in the current marketplace, it is fair to assumethat the major cost of these catalysts would come from the procurementof these two rare elements. Comparison between the production rates for1.0 wt % Pt-P25 and 1.0% Pt-anatase to that of the 0.17 wt % Ausupported on P25 or anatase nanoparticles can be made by normalising forthe amount of precious metal present in the catalyst instead of by thetotal mass of the catalyst; these H₂ production rates normalised byprecious metal mass are shown in FIG. 9.

This comparison shows the increased efficacy of Au/TiO₂ compared totypical Pt—TiO₂ photocatalysts. For the anatase-supported series, Au₁₀₁calcined under O₂ and H₂ is ^(˜)20 times more effective than Pt-anatasefor the same amount of precious metal present in the catalyst, while forthe P25 series, heat-treated Au₉ is ^(˜)66 times more effective thanPt-P25 for the same amount of precious metal present in the catalyst.Even the least effective Au catalyst in the series, untreated Au₈ onpure anatase, is more effective than both Pt-P25 and Pt-anatase whencompared using this normalisation scheme.

Conclusions from Examples 1 to 11

Benchmark photocatalytic water-splitting experiments were undertakenusing Pt-P25 and Pt-anatase nanoparticles to ensure the newly designedexperimental apparatus was performing adequately. During these studies,it was discovered that there Is a decrease in the photocatalyticperformance of samples when repeating experiments using the samecatalyst material. Further preliminary studies of Au/TiO₂ found asimilar effect of performance degradation. This degradation in theperformance of samples was accompanied by a colour change in thesamples. The degradation and colour change of samples was attributed toaccumulation of carbon deposits during the oxidation of organiccompounds, and may be related to the known photo-induced agglomerationeffects of ambient light on the Au₉ and Au₁₀₁ clusters evidenced byXANES and HRTEM data. Improved performance was also observed for allsamples prepared in the reaction cell with 12 hours of exposure tovacuum, compared to relatively short vacuum exposure times of 10minutes.

The production of H₂ from photocatalytic water-splitting experiments wasaccompanied by the production of CO₂ and consumption of O₂. The CO₂by-product arises from the well-known capacity for TiO₂ to photo-oxidiseorganic contaminants, and consumes the stoichiometrically evolved Ozfrom the water-splitting reaction throughout the experiment. The sourceof carbon in the reaction cell is most likely from unavoidableadventitious carbon that is present in all vacuum systems and samplesexposed to atmosphere, in addition to the possible contribution from oilback-streaming from the rotary pump. Various carbon based sealantmaterial used in the reaction cell and adsorbed CO that is difficult toevacuate during sample preparation may also contribute to the source ofcarbon.

O₂ present in the reaction cell at the beginning of the experiment dueto low vacuum is likely rapidly consumed by quenching defect stateswithin the TiO₂ nanoparticles and by photo-adsorption of O₂ to the TiO₂surface over the initial hour of experiments. This initial 02 presencecould also include O₂ molecules adsorbed to the TiO₂ surface at ambienttemperature, or those adsorbed to the walls of the reaction cell. Theformation of surface O₂ ⁻ and O₃ ⁻ species during this period bymolecular O₂ likely behaves as electron traps or hole scavengers afterphoto-excitation, increasing electron-hole separation, which couldexplain the decrease in both H₂ and CO₂ production after the excess O₂in the reaction cell is consumed.

Example 12 Ru₃ Nanoclusters on Titania

Ru clusters have interesting properties when it comes to catalysis andthese are mostly unexplored. The materials explored in this example areligand stabilised clusters on a titania support. All the experimentswere conducted at 2 bar with a 4:1 ratio of H₂ to CO₂. All Ru clusterswere loaded at 0.17% on titania.

For Ru₃ a series of experiments were performed testing different Hacalcination temperatures. A reactor was filled to 1.5 bar with H₂, andheated to the target temperature at a rate of 10° C. per min. A seriesof different reaction temperatures were tested, a series of differentmasses on a substrate were tested and finally a series of varyingevacuation techniques.

FIG. 10 shows the production rates at varying calcination temperaturesfor CO, methane and ethane (note the axis is on the right for CO). Foreach of these samples XPS data was also gathered, and saw partialoxidation at higher calcination temperatures. The species of Ru are yetto be clearly identified but it is known from XPS that a RuO forms.Initial tests were carried out over several hours with Ru nanoclusterson titania. The Sabatier mix used was 3:1 ratio of H₂ to CO₂ with theaim to produce methanol.

After identifying a consistent calcination temperature, a series ofsamples at different reaction temperatures were explored. Surprisinglyrelatively low temperatures are required, contrary to literature wherevalues of 300° C. and above are reported. FIG. 11 shows there is a clearsaturation after 220° C., ideal temperature being 250° C.

Example 13 Ru₄ Nanoclusters on Titania

At a calcination temperature of 200° C. and a reaction temperature of250° C. the gases produced are 379 μmolh⁻¹ g⁻¹ of methane, 4649 μmolh⁻¹g⁻¹ of CO and 149 μmolh⁻¹ g⁻¹ of ethane.

Ruthenium nanoparticles at a 3% loading produced in the range of2000-3000 μmolh⁻¹ g⁻¹ of methane, but had 20 times more Ru than thecluster samples. Production rate normalised to Ru mass shows that Ruclusters out-perform the ruthenium nanoparticles by almost 4 times asmuch.

To compare Ru₄ with Ru₃ nanoclusters, the same series of hydrogencalcinations was completed at the same temperature. These are plotted inFIG. 14. CO production was similar for both, but Ru₃ had a generalbetter production rate for methane and ethane.

Example 14 the Effect of the Thickness of the Photocotalyst in Step (b)

Experiments were performed in order to determine the effect of thethickness of the deposited photocatalyst on a silicon wafer substrateand whether it had an effect on the hydrocarbon gas production rates.For this experiment the weight of catalyst on the wafer was determined.FIG. 12 shows that lower loadings on the Si wafer produced more CO,decreasing with weight. Whilst methane was pretty stable throughout, yetstill increasing slightly as the mass increased.

Prior to this, all samples were left overnight in the reactor pumpedovernight using a rotary vane pump; reaching pressures of approx 1.102mbar. A series of different conditions were tested, as shown in FIG. 13.After each evacuation, the samples were calcined at 200° C. in H₂ at 1bar. The reaction temperature was 250° C.

Example 15 Photocatalytic Studies of AuRu₃ Deposited Upon Anatase TiO₂

The Ru₃(μ-AuPPh₃)(μ-Cl)(CO)₁₀ cluster was deposited upon anatase TiO₂,(hereinafter referred to as “AuRu₃—TiO₂”) and was evaluated forphotocatalytic solar fuel production in the gas-phase, using aheterogeneous batch reactor apparatus. H₂ and methane were detected asthe major products of these reactions, with longer-chain hydrocarbons upto C₄ species observed as minor products under certain conditions.

Example 15.1 Synthesis of Ru₃AuPPh(μ-Cl)(CO)₁₀

The Ru₃AuPPh₃(μ-Cl)(CO)₁₀ was synthesised as follows along the lines ofthe technique described in the paper entitled Synthesis and StructuralCharacterization of a New Ruthenium-Gold Cluster Complex:Ru₃AuPPh₃(μ-Cl)(CO)₁₀, Inorganic Chemistry, Vol. 23, No. 5, (1984). Intypical synthesis, 310 mg of Ru₃(CO)₁₂ and 240 mg of AuPPh₃Cl weredissolved in 50 mL of dry dichloromethane. The solution was stirred andrefluxed (at 50° C.) under N₂ atmosphere overnight. After the reactionmixture was cooled to room temperature, silica gel 60 was added. Thesolvent was removed under vacuum for about an hour. The reaction mixturewas chromatographed on silica gel 60. Elution with toluene-petroleumether (1-1) afforded yellow band of Ru₃(CO)₁₂. Further elution with puretoluene afforded violet band of Ru₃AuPPh₃(μ-Cl)(CO)₁₀. After solventremoval in vacuo, dichloromethane-hexane (1-5) was added. The solventwas removed under reduced pressure using rotary evaporator to obtain thecrystals of the violet solution.

Drying Anotase

A 12 g (12.0749 g) of anatase was dried in vacuo at 200° C. for 5 hourswith stirring. After cooling to room temperature, dry anatase was keptin desiccator overnight. A 1.9% weight loss was found according tomoisture content.

Preparation of Ru₃AuPPh₃(μ-Cl)(CO)₁₀ Stock Solution

A 300 mg (302.13 mg) of Ru₃AuPPh₃(μ-Cl)(CO)₁₀ crystals was dissolved insmall amount of dichloromethane. The solution was transferred into 25-mLvolumetric flask following by making volume up to 25 mL by addingdichloromethane to obtain Ru₃AuPPh₃Cl(CO)₁₀ stock solution.

Deposition Ru₃AuPPh₃(μ-Cl)(CO)₁₀ on Anatase

X grams of dry anatase (see Table below) was suspended in 20 mL ofdichloromethane in a Schlenk tube. After vigorously stirring (750 rpm)under N₂ for 30 min, a Y μL of Ru₃AuPPh₃Cl(CO)₁₀ stock solution wasinjected into the Schlenk tube. The solution was stirred (750 rpm) atroom temperature under N₂ for 90 min. The solvent was carefully removedunder vacuum for around an hour to obtain Ru₃AuPPh₃(μ-Cl)(CO)₁₀deposited on anatase. The final catalyst was sonicated and thentransferred into a sealed vial.

Y, Ru₃AuPPh₃Cl(CO)₁₀ % metal loading X, Anatase (g) stock solution (μL)0.08 1.1990 (1.19978) 172 0.17 1.1980 (1.19843) 366 0.35 1.1958(1.19522) 754 0.50 1.1940 (1.19435) 1078 0.75 1.1910 (1.19137) 1616 1.001.1880 (1.18806) 2155 1.50 1.1820 (1.18252) 3233 2.00 1.1760 (1.17650)4311 5.00 1.1400 (1.14039) 10776

Example 15.2 Photocatalytic Benchmarks and Comparisons

AuRu₃/TiO₂, Pt/TiO₂ & Bare TiO₂

To assess the relative photocatalytic activity of AuRu₃/TiO₂ towards CO₂reduction, two other materials were tested as benchmarks. Bare anataseTiO₂ nanoparticles (Sigma-Aldrich) were tested as-purchased, as well as1 wt % Pt nanoparticles deposited upon P25 TiO₂. Preliminary testingindicated that pre-treatment and reaction temperatures of 200° C. and aP_(CO2):P_(H2O) ratio of 3 were optimal for solar fuel production, andso these conditions were used for testing all samples. These willhereafter be referred to as “standard conditions”.

H₂ and methane were detected as major products, as well as trace amountsof C₂-C₃ alkane and alkene species. To ensure that the source of theseproducts was indeed the reagent gases and not other carbonaceouscontaminants, control reactions were conducted (i) without catalyst, inthe presence of UV irradiation and reagent gases; (ii) in the absence ofUV Irradiation, with catalyst and reagent gases, and (iii) with catalystunder UV irradiation, but with argon buffer gas used in place of thereaction mixture. The former two tests yielded negligible amounts of theproducts of interest here, however the third blank test gave off tracelevels of C₁-C₃ hydrocarbons. Further investigation showed that theseresidual hydrocarbon levels scaled linearly with the mass of catalystused, and is likely due to the photo-induced breakdown ofsurface-adsorbed advantageous hydrocarbons or ligands. This backgroundhydrocarbon production was normalized to total catalyst mass, andsubtracted from all subsequent photocatalytic tests.

Photocatalytic production rates of methane and hydrogen by anatase TiO₂,AuRu₃/TiO₂ and Pt/TIO₂ are shown in FIG. 16A. Production rates of minor,longer-chain hydrocarbon products are then shown in FIG. 16B. Depositionof the AuRu₃ duster improves the turnover of both major and minorhydrocarbon products relative to bare anatase, with methane productionincreasing by ^(˜)3× and ethane by a factor of two. In the case of theminor hydrocarbon products, the large uncertainties make Interpretationof this data difficult. These errors are predominantly due to theextremely low levels of these products generated (^(˜)10-100 ppb),giving poor signal-to-noise ratios in the GC-FID. However, it can besaid with confidence that generation of C₃ products propane and propeneis greater for the AuRu₃-deposited sample than the bare titania, asthese production rates do not agree even under these large experimentalerrors. Additionally, negligible H₂ production is observed over the bareanatase substrate, whereas the AuRu₃/TiO₂ system generates 68.5 μmolhr⁻¹ g⁻¹.

When comparing the activities of AuRu₃/TiO₂ with Pt/TiO₂, the lattershows higher production rates for both methane and hydrogen. This isunsurprising considering the greater content of co-catalyst onplatinized sample than on the cluster-deposited sample (vide infra).However, greater amounts of saturated, longer-chain hydrocarbon productsethane and propane are generated by AuRu₃/TiO₂ than Pt/TiO₂. The samecannot be said for unsaturated products; Pt—TiO₂ generates more ethenethan AuRu₃/TiO₂, and the levels of propene generated by these twocatalysts agree under experimental error. Evidently, Pt/TiO₂ has a muchhigher selectivity for the formation of unsaturated hydrocarbon productsthan AuRu₃/TiO₂.

To account for the different loadings of co-catalyst upon AuRu₃—TiO₂ andPt—TiO₂, FIGS. 17A and 17B show the same production rates discussedabove, but instead normalized to the total precious metal content (Pt orAu/Ru) deposited upon the TiO₂ nanoparticles. As can clearly be seenwhen compensating for total co-catalyst content, the cluster-basedAuRu₃/TiO₂ out-performs Pt/TiO₂ in the generation of all productsdetected here. As platinum nanoparticles can be highly-activeco-catalysts for CO₂ photo-reduction this is extremely promising forpotentially further improving the efficiency of these reactions by useof sub-nanometer clusters instead of nanoparticles. However, as thecluster and nanoparticle co-catalysts compared here have entirelydifferent elemental compositions, it must be acknowledged that theimproved activity of AuRu₃/TiO₂ may be due to selection of a moreappropriate metal for this reaction as well as (or instead of) thebenefits of sub-nanometer co-catalysts over nanoparticulate species.

Example 16 a Comparison Between the Catalysts: AuRu/TiO₂, Ru₂/TiO₂ andRu₄/TiO₂

Both the Ru₃(CO)₁₂ precursor of AuRu₃ and a H₄Ru4(CO)₁₂ clusterdeposited upon TiO₂ have previously been characterized as catalysts forwater-splitting, ethene hydrogenation and Sabatler CO₂ reduction.Therefore, these clusters were deposited upon anatase TiO₂ and testedfor CO₂ photo-reduction in the same manner as AuRu₃/TiO₂. FIGS. 18A and18B compare the photocatalytic activities of these three cluster speciesdeposited on TiO₂ for solar fuel production under UV Irradiation.

Methane and H_(z) are detected were major products across all threespecies, with C₂-C₃ hydrocarbons as minor products. Both Ru₃—TiO₂ andRu₄—TiO₂ also gave off low amounts of CO under UV irradiation; however,the quantities of this varied between scans (potentially due tode-ligation of carbonyl ligands convoluting this signal), and so havebeen excluded from analysis here. As is clearly evident in FIG. 19,AuRu₃/TiO₃ exhibits the highest rate of H₂ production out of all threecluster-based systems, with Ru₃/TiO₂ and Ru₃/TiO₂ giving very similarproduction rates ^(˜)3× lower than this. In terms of methane production,Ru₃/TiO₂ again gives the lowest catalytic efficiency, with Ru₄/TiO₂ andAuRu₃/TiO₂ yielding higher production rates. Within the experimentalerrors of these measurements, both AuRu₃/TiO₂ and Ru₄/TiO₂ givecomparable methane production rates. Similar trends are observed formost of the minor hydrocarbon products, with large experimentaluncertainties in production rates of ethane, propane and propene againpreventing further conclusions from being drawn. The exception to thisis ethene, for which Ru₄/TiO₂ generates appreciably greater amounts thaneither of the other two photocatalysts. However, hydrocarbon productionrates for both AuRu₃/TiO₂ and Ru₄/TiO₂ are greater than for Ru₃/TiO₂.From the above results, it is evident that addition of a single metalatom to the Ru₃ cluster—either gold or ruthenium—improves its overallrate of solar fuel production when supported upon anatase titania.However, substitution of gold for a ruthenium atom reduces the H₂generation rate to that of Ru₃—TiO₂. Therefore, it is likely that theimproved H₂ production rate of AuRu₃/TiO₂ is due to electronicinteractions of the gold atom, possibly shifting the cluster'selectronic structure to favor H⁻ reduction or OH oxidation. As theimprovement in hydrocarbon production occurs regardless of the addedelement, this is less likely to be a purely electronic effect, and couldmore plausibly be due to the increased cluster size allowing for moreeffective reagent binding.

To assess the potential competitive nature of the water-splitting andCO₂ photo-reduction reactions recorded here, these three cluster-basedmaterials were also tested for water-splitting, i.e. in the absence ofCO₂. FIG. 19 shows the relative H₂ production rates from these materialsobserved under these conditions, alongside the same production rateswith CO₂ present. Within the experimental uncertainties here, all threeclusters exhibit comparable hydrogen production rates both with andwithout CO₂ present in the reaction mixture. This indicates that thepresence of CO₂ does not hinder the water-splitting reaction for thesematerials in any way; or conversely, that CO₂ does not effectivelycompete with water for reduction at the photocatalyst surface.

Example 17 Optimising Pre-Treatment Temperature for Au—Ru Catalysts

Heterogeneous catalysts and photocatalysts are generally pre-treated insitu before testing, in order to remove advantageous hydrocarbons andother surface-adsorbed species, or to open up catalyst active sites byremoval of ligands. Many different techniques for this can be undertakenfor example including ozone treatment, calcination in O₂ or H₂, andheating under a flow of inert gas. However, the inventors workdemonstrates that many of these treatments have damaging effects uponclusters deposited upon TiO₂, often causing aggregation to largernanoparticles. This is undesirable in developing cluster-based catalyticmaterials, as it removes the size-specific nature of the clusterco-catalysts and complicates the correlation of catalytic activity toparticle size. Hence, selection of an appropriate pre-treatment whichremoves adsorbed contaminants while still retaining intact clusters uponthe surface for these materials is paramount.

Heating under vacuum was selected for catalyst pre-treatment, as it wasshown to have the least aggregative properties of material treatmentsstudied. All photocatalytic materials discussed above were heated to200° C. while pumping under vacuum for 20 minutes. However, a range oftemperatures from 50-250° C. (the limit of the apparatus) were alsotested for AuRu₃/TiO₂. FIGS. 20A and 20B show the dependence ofphotocatalytic activity upon this pre-treatment temperature for majorand minor products, respectively. It should be noted that throughoutthese experiments, the reaction temperature was kept the same as thepre-treatment temperature. This was done to ensure that no samples weretested at higher temperatures than they were pre-treated, such that anyobserved change in chemical state could be ascribed to the treatment andnot the reaction. Using constant, lower reaction temperatures of 50-100°C. could also have achieved this; however as can be seen below, reactingat these temperatures yielded little to no activity. Therefore, it mustbe acknowledged that the resultant photocatalytic production rates shownin FIGS. 20A & 20B are convoluted also by effects of reactiontemperature. The dependence upon reaction temperature will be discussedin Example 18.

Despite this aforementioned convolution of pre-treatment and reactiontemperature, an optimal pre-treatment temperature of 200° C. is evidentfor all products of interest. Production of methane and C₂₋₃hydrocarbons peak at this temperature, with near-baseline production atmost other temperatures. Hydrogen production displays a differentbehaviour, increasing almost linearly from 50-200° C., before thendecreasing slightly beyond 200° C. Therefore, a pre-treatmenttemperature of 200° C. appears to be justified here, as it gives optimalactivity towards all products of interest.

Example 18 Optimising Reaction Temperature for Au—Ru Catalysts

To de-couple the effects of pre-treatment and reaction temperature, aseries of photocatalytic tests on AuRu₃/TiO₂ were run over a range ofreaction temperatures, while keeping a constant material pre-treatmenttemperature. 200° C. was chosen as the standard temperature forpre-treatment due to the results from Example 17. FIG. 21A shows thedependence of major product turnovers on reaction temperature, whileFIG. 21B shows the same for minor hydrocarbon products detected.Notably, in addition to methane and H₂, carbon monoxide was alsodetected as a major product by GC analysis at some reactiontemperatures. As the CO parent ion at m/z=28 overlaps with afragmentation peak of CO₂, RGA analysis could not be used to verify thisquantification of CO.

Peak generation rates of most products of interest were observed at areaction temperature of 150° C. At near-ambient temperatures of 50° C.,no appreciable levels of hydrocarbon products are detected, with only H₂and CO observed in low amounts. The first hydrocarbons are detected at atemperature of 100° C., corresponding to a decrease in CO production anda very slight increase in H₂ production. Most products follow a verysimilar trend, increasing in yield from 50-150° C., before decreasing onfurther raising the temperature to 200° C. The exceptions to this are C₃products such as propane, which have such large experimentaluncertainties that no reasonable conclusions can be made; and CO, whichshows a decrease from 50-100° C., before peaking at 150° C. like allother products. Several previous works in this field have postulatedthat CO may be an intermediate species in the reduction of CO₂ tohydrocarbons. Hence, the more complex relationship with reactiontemperature that CO exhibits here may be due to it being consumed toproduce hydrocarbons such as methane or ethane.

A distinctly non-linear relationship with temperature is observed forall products of this photocatalytic reaction. The reaction may belimited by adsorption-desorption effects upon the TiO₂ surface, wherethe rate-limiting step is desorption of products at lower temperatures,and adsorption of reagent molecules at higher temperatures. Reacting at150° C. may achieve an optimal equilibrium between reagent adsorptionand product desorption. At lower temperatures, poor desorption ofproducts or intermediates from water reduction could simultaneouslylimit the H₂ production rate and proton transfer to CO₂. As the reactiontemperature then increases these reduced states of water would then bemobilized and more readily desorbed, allowing for formation of C—H bondsand giving greater H₂ production rates. However, on rising above 150° C.the limiting factor could then become reagent adsorption, with theexcess thermal energy in the system causing molecules to desorb from theTiO₂ surface before completing photocatalytic transformations and hencedecreasing overall production rates.

Conclusions from Examples 15 to 18

When comparing AuRu₃/TiO₂ to Ru₃(CO)₁₂ or H₄Ru(CO)₁₂ clusters depositedupon titania, the bimetallic-deposited species shows the greatestaffinity for H₂ production from water, while both M₄-based clusters showimproved hydrocarbon turnover when compared to Ru₃—TiO₂. Optimalturnover is observed when the photocatalyst was pre-treated under vacuumat 200° C. and reacted at 150° C. and higher partial pressure ratios ofH₂O to CO₂ improve hydrocarbon production rates. Optimal production ofCO and H₂ was observed at a reagent ratio of 1:1, and CO₂:H₂O ratios inthe range of 0.5-4 gave peak hydrocarbon production.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the Invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as Illustrative and notrestrictive.

In the claims which follow and in the preceding description of theinvention, except where the context requires otherwise due to expresslanguage or necessary implication, the word “comprise” or variationssuch as “comprises” or “comprising” is used in an inclusive sense, i.e.to specify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments of theinvention.

The invention claimed is:
 1. A method for the production of hydrocarbonsor substituted hydrocarbons, the method comprising the steps of:contacting a catalyst with water and carbon dioxide in the presence oflight in order to photocatalyse: (i) the splitting of at least some ofthe water into hydrogen and oxygen; and (ii) the reaction betweenhydrogen and carbon dioxide to produce at least one of a hydrocarbon anda substituted hydrocarbon; wherein the catalyst comprises at least goldand ruthenium, in the form of at least one nanocluster supported by asupport substrate.
 2. The method according to claim 1, wherein supportsubstrate is selected from the group comprising graphene, graphite,carbon black, nanotubes, fullerenes, zeolites, carbon nitrides, metalnitrides and or oxides including zinc oxide or titanium oxide.
 3. Themethod according to claim 1, wherein the gold and ruthenium nanoclusterhas at least one Au—Ru bond having a distance in the range of from about2.5 to 3.0 Å.
 4. The method according to claim 1, wherein the gold andruthenium nanocluster comprise an average cluster size less than about 2nm.
 5. The method according to claim 1, wherein the catalyst comprises afirst photocatalyst and a second catalyst, the method comprising thesteps of: a. contacting a first photocatalyst with water in the presenceof light in order to photocatalyse the splitting of at least some of thewater into hydrogen and oxygen wherein the photocatalyst comprises goldnanoclusters supported by a titanium dioxide substrate; and b.contacting a second catalyst with a gas stream comprising carbon dioxideand at least some of the hydrogen produced from step (a) in order tocatalyse the reaction between the hydrogen and carbon dioxide to produceat least one of a hydrocarbon and a substituted hydrocarbon; whereinsaid second catalyst comprises ruthenium nanoclusters supported by atitanium dioxide substrate.
 6. The method according to claim 5, whereinthe titanium dioxide support substrate comprises titanium dioxidenanoparticles.
 7. The method according to claim 5, wherein the goldnanoclusters are selected from (Ph₃PAu)₃OBF₄, [(AuPPh₃)₃O]PF₆,Au₈(PPh₃)₄Cl, Au₆(PPh₃(BF₄)₂, Au₆(PPh₃MNO₃)₂, Au₆(PPh₃)₆(PF₆)₂,Au₈(PPh₃)₈(NO₃)₂, Au₈(PPh₃)₇(NO₃)₂, Au₉(PPh₃)₈(NO₃)₃, Au₁₀(PPh₃)₅(C₆F)₄,Au₁₁Cl₃ {(m-CF₃C₆H₄)₃P}₇, Au₁₁(PPh₃)₇(PF₆)₃, [Au₁₃(PMe₂Ph)₁₀Cl₂](PF₆)₃,Au₁₃(PPh₃)₄[S(CH₂)₁₁(CH₃)]₄, [Au₁₃(PPH₂CH₂PPH₂)₆](NO₃)₄,Au₅₅(PH₂PC₆H₄SO₃Na.2H₂O)₁₂Cl₆, Au₅₅(PPh₃)₁₂Cl₆, Au₁₀₁(PPh₃)₂₁Cl₅, where“Ph” is phenyl and “Me” is methyl.
 8. The method according to claim 5wherein the ruthenium nanoclusters comprise either Ru₃ or Ru₄.
 9. Themethod according to claim 1 wherein the percentage coverage of thesupport substrate with nanoclusters is at least about 0.1, 0.5, 1, 2, 3,4, 5 or 10% as a percentage of the total available surface area.
 10. Themethod according to claim 5 wherein the method further includes the stepof pre-treating the photocatalyst and/or catalyst prior to use; and thepre-treatment includes exposing the photocatalyst and/or catalyst toelevated temperatures under a vacuum, or in the presence of hydrogen oroxygen.
 11. The method according to claim 1 wherein the amount ofmethane produced is at least about 350, 450, 550, 1000, 2000, or 5000μmol hr⁻¹ g⁻¹ cm⁻².
 12. The method according to claim 5, whereinsubstantially all of the hydrogen produced at step (a) is used in step(b).
 13. The method according to claim 1 wherein the hydrocarbon ismethane.
 14. The method according to claim 1 wherein the substitutedhydrocarbon is methanol.
 15. The method according to claim 1 wherein theamount of hydrogen produced by the method is at least about 15, 50, 80,100, 150, 200, 250, 350, 450, 550, 1000, 1500, 2000 or 5000 μmol hr⁻¹g⁻¹ cm⁻².